The present invention relates to femoral parts of a knee prosthetic. It relates notably to femoral parts of a knee prosthetic of the type known as comprising three compartments, i.e. two parts which define the condyles and are separated by a trochlea surface.
These prosthetic femoral parts are implemented to co-operate with an insert often made of a material such as polyethylene, laid onto the tibia plate of a tibia part, with said co-operation taking place by means of the external surfaces of the condyles of the femoral part rolling and sliding on the upper concave surfaces of the insert.
A prosthetic femoral part of this type is known from the prior art, notably from the international patent application WO92/03108 of British Technology plc. With reference to a longitudinal cross-section, i.e. with reference to a section parallel to the longitudinal axis of the femur and to the longitudinal axis of the tibia with the knee flexed, this prosthetic femoral part features two circular arcs, with the first of said circular arcs corresponding to the trochlea surface and thus defining the trochlean trajectory. This radius will be designated further with the reference R1. The second circular arc of this longitudinal cross-section corresponds to the two condyles. This radius will be designated further with the reference R2.
According to the prior art, the value of radius R1 is slightly greater than the value of radius R2, generally according to a 1.2 ratio. These types of femoral parts, when used in co-operation with upper concave surfaces with a corresponding spherical shape or cylindrically circular shape, ensure a perfect congruence between the surfaces of the condyles and the surfaces of the tibia insert throughout the flexion range, i.e. between 0xc2x0 and 130xc2x0. However, this type of knee prosthetic, according to the prior art features the problem that the constraints to which the ligaments and notably the upper crossed ligament, are submitted in the flexion range between 80xc2x0 or 90xc2x0 and 130xc2x0 are very elevated.
The present invention aims to remedy this problem by means of a prosthetic femoral part which allows to decrease the constraints applied to the ligaments in the flexion range between 80xc2x0 or 90xc2x0 and 130xc2x0.
According to the invention the femoral part of a knee prosthetic, including two condyles and a trochlea surface within which a trochlean trajectory is defined in a longitudinal section, i.e. with reference to a section parallel to the longitudinal axis of the tibia and to the longitudinal axis of the femur with the knee flexed, the trochlean trajectory being a circular arc and at least part of the condyles also being a circular arc, is characterised in that in a longitudinal section the condyles comprise two circular arcs, i.e. an intermediary circular arc and an extremity circular arc. The intermediary circular arc extends between the circular arc which defines the trochlean trajectory and the extremity circular arc with the value of the curvature radius of the extremity circular arc being lesser than the value of the curvature radius of the intermediary circular arc, the intermediary circular arc extending in an angle of 80xc2x0 to 90xc2x0 and the extremity circular arc extending in an angle of 40xc2x0 to 50xc2x0.
A perfect congruence is therefore obtained with the concave surfaces of the tibia insert having a curvature radius more or less equal to the curvature radius of the intermediary circular arc in a flexion range of 80xc2x0 to 90xc2x0. However, for an increased flexion, i.e. beyond 80xc2x0 or 90xc2x0, the congruence decreases such that some looseness appears between the femoral part and the tibia insert and said looseness allows for the decrease of the constraints to which the ligaments, notably the upper crossed ligament, are submitted. Knee prosthetics including a femoral part of this type can be used without harming the upper crossed ligament in the long-term.
According to a preferred method of implementing the invention, the ratio of the intermediary circular arc over the extremity circular arc stands between one and 2.5 and preferably between 1.2 and 1.8.
The invention also relates to a knee protection including a femoral part such as previously defined and a tibia insert laid onto a tibia plate, with said tibia insert including an upper concave surface which co-operates with the external surface of the condyles, as the curvature radius of the external surface of the insert is sensibly equal to the curvature radius of the intermediary circular arc.
The invention will now be described by way of example only with reference to the accompanying drawings.